Radiotherapy is carried out as one of the therapies for cancer. The radiotherapy is a therapy involving suppressing growth of cancer cells and killing cancer cells through irradiation with an X-ray, a γ-ray, or the like. The radiotherapy is considered to be an important measure for palliative medicine, because it is generally a local therapy, is less invasive to the whole body, and hence allows a burden on a patient to be reduced.
In the radiotherapy, an external irradiation method is a method involving irradiating a focus as a target from the outside of the body, and uses an X-ray, a γ-ray, an electron beam, a particle beam, or the like. In the external irradiation method, it is necessary to consider a direction of radiation irradiation so that a sufficient dose of radiation reaches a tumor and a normal tissue around the tumor does not receive a high dose of radiation. Radiation irradiation may be carried out from one direction, or may be carried out from numerous directions. A therapy for a tumor present in a deep portion in the body requires strong radiation irradiation, which causes problems in terms of side effects.
The side effects of the radiotherapy on a normal tissue basically occur in an irradiated region. A response of the normal tissue is exemplified by mucosal or epithelial cell damage, which develops during irradiation, and interstitial cell or vascular endothelial cell damage, which develops six months to several years after completion of the therapy. The radiotherapy causes various side effects depending on an irradiation dose, an irradiation site, a patient's age and systemic condition, and the like. There is a demand for radiotherapy capable of effectively killing cancer cells while reducing the side effects.
In recent years, a photocatalytic action of titanium dioxide or the like has been utilized in various fields and has attracted attention. The photocatalytic action is based on an oxidizing ability of a reactive oxygen species to be generated by irradiating titanium dioxide or the like with light having a short wavelength of 380 nm or less. The photocatalytic action has been utilized in a decomposition treatment of hazardous chemical substances such as an environmental hormone and in killing and suppressing growth of hazardous microorganisms, and further, its applied research toward the therapies for cancer is proceeding (Non Patent Literatures 1 to 4).
It has been disclosed that a titanium dioxide composite, which is obtained by modifying titanium dioxide with a hydrophilic polymer and further immobilizing thereto a molecule having a specific binding ability to a molecule of interest, has an action of decomposing cancer cells and the like through a photocatalytic action (Patent Literature 1). Such titanium dioxide composite exerts a photocatalytic action through ultraviolet ray irradiation.
Further, it has been confirmed that titanium dioxide generates a hydroxyl radical at a high concentration when subjected to ultrasonic irradiation (titanium dioxide/sonocatalytic method). There is a disclosure of a composite particle obtained by coating titanium dioxide with polyacrylic acid and binding a liver cell recognition protein pre/S2 to the titanium dioxide. Such composite particle is capable of specifically targeting liver cancer cells and exerts actions of killing/damaging cancer cells through ultrasonic irradiation (Patent Literature 2).
Although it has been demanded that the photocatalytic action be utilized in the radiotherapy, no photocatalytic material which can be put into practical use has been found yet, because there are problems in terms of an irradiation dose and the like.